3rd edition
SENSO - News
Recent Projects: Innovative solutions for rotor retaining rings and damper
system on 150 MVA air-cooled turbo generator
Karlsruhe/Germany. EnBW Kraftwerke AG is a subsidiary of EnBW AG
(Energy Baden-Württemberg AG), the third-largest utility in Germany.
With an output of 1,260 MW, the Rhine Ports Steam Power Plant in
Karlsruhe is one of the largest conventional power plants of EnBW
Kraftwerke AG.
Unit 4 of this plant was modernized and converted into a combined cycle
process in 1998, rendering it one of the most efficient power generating
stations in Germany (57%). In the course of this so-called “repowering”
project, the original steam turbine was reconfigured to provide a seriesconnection to a new gas turbine, with the 600°C res idual heat generated
from the gas turbine used to feed the boiler of the steam turbine (HRSG).
Removing of retaining rings
SENSOPLAN was commissioned with remedying the slot coupling problems in the air-cooled steam turbine
generator with a rating of 150 MVA (see also SENSOPLAN Service article on the next page). While on-site,
SENSOPLAN engineers also noted a knocking noise emanating from the rotor.
Further investigation revealed that loose supports and spacers in the
rotor end winding, slipping up and down during turning-gear operation,
were causing the knocking sound. To correct this condition, the rotor
was first removed and transported to the repair workshop. However,
upon commencing the rotor disassembly to perform the end winding
repairs, the rotor retaining rings could not be removed in the standard
fashion. Examination revealed that the damper ring of the interior
damper system had expanded during operation to such an extent that
the damper finger segments were blocked and the bayonet locks on the
retaining rings, typical on these machines, could no longer be opened.
Rotor end winding
In short order, SENSOPLAN engineered the right solution, providing a
removal process that preserved the integrity and functionality of the
rotor retaining rings and the related components (e.g. damper rings).
Upon removal of the retaining rings, additional extensive damage to
the damper system and the end winding support was found, including
burn marks in the wedges, damper finger segments and retaining
rings, dirty and damaged end winding insulation, broken spacers, and
rotor notching caused by the loose end winding supports. It was now
important to respond promptly to these unexpected findings. How
could a lengthy outage extension, which a renovation of damping
system and end winding supports would entail, be avoided, while at the
Re-installation of the rotor
same time ensuring the safe operation of the rotor until a scheduled
repair could be arranged?
An ideal temporary solution was identified by the SENSOPLAN engineers. The rotor was cleaned thoroughly,
and the damper system was repaired and made serviceable for a limited time with carefully designed and
targeted modifications. Additionally, the end winding support was reconditioned, sealed and secured to the
best extent possible. Following the careful and precise reassembly of the rotor, it was high-speed balanced
and overspeed tested, with the vibration behavior and electric values tested under extreme conditions.
Despite the great degree of additional work, the rotor was serviced and made fully functional within the
originally scheduled outage window. The operator now has approx. two years to properly plan the permanent
repair, including procurement of all necessary engineering and craft services, tooling, and replacement
components. With optimal preparation, a complete repair can be carried out speedily and on schedule.
SENSONEWS © SENSOPLAN 11/2006
SENSOPLAN AG Gewerbestr. 6
79801 Hohentengen a.H., Germany
Phone +49 7742 9298-0
Fax +49 7742 9298-10
SENSOPLAN Inc. 1020 e Wall Street
Mt. Pleasant, SC 29464, USA
Phone +1 843 329 0275
Fax +1 843 329 0956
Please visit us at www.sensoplan.com or send us a note to info@sensoplan.com (Europe) or sales@sensoplan.com (USA)
SENSO - News
Applications: Comprehensive condition analysis of 300 MVA motor
generators
Wehr/Germany. Pumped-storage hydropower stations supply
electricity to meet peak power requirements. By reacting quickly,
they are able to adapt to the actual demand for power, but as a
result are subject to high loads due to the frequent changes in mode
of operation. The underground hydroelectric power station Wehr
located in southern Germany run by the Schluchseewerk company
is one of the largest pumped-storage plants in the world. There
SENSOPLAN conducted condition monitoring and analysis of the
four motor generators: an essential requirement for safe operation
and total availability.
Motor generator prepared for loop test
The Wehr hydroelectric power station is located 400 meters below the surface of the earth in an
underground cavern and is comprised of four hydroelectric generator sets. Each of the generator sets
is 30 meters long and includes turbine, pump and electric machine (motor generator) with a rating of
300 MVA. A special feature of such power plants is the motor generator: During periods of low
electricity demand, this machine operates as a motor and drives the pump, which in turn pumps water
into the storage reservoir located above. To cover peak electricity demand on short notice, water is
released from the reservoir to power the turbines and thus operate the motor generator as generator.
To sustain the safe operation of the plant, the customer instructed
SENSOPLAN to conduct a comprehensive condition analysis of
the motor generators. Because of their service performance on
other large machines in conventional, nuclear and hydro power
generation, the plant owners knew SENSOPLAN to be a reliable
partner with the specific know-how and equipment for just such
an analysis. A comprehensive analysis includes getting to know
the “life story” of the machine and finding out what the special
characteristics are and where the particular problems lie.
Thermography of lamination stack
One of the special characteristics of these hydro-electric
machines is that the stator winding, as well as the back of the
lamination stack, is cooled with ultra-pure water. Following a
thorough visual inspection of all the accessible and visible
components, various electrical tests were performed. The partial
discharge test is used to evaluate the insulating behavior of the
windings, so that, for example, the ageing of the insulation
system can be determined. SENSOCORE HI Check up is a high
induction loop test under near operating conditions that reveals
possible hot spots in the aged lamination stack. Thanks to
Finite-Element-Analysis of stator frame
SENSOPLAN’s test container, this test could be performed on
site. It was, therefore, not necessary to go through the complex
procedure of removing the rotor, as the equipment and expertise for performing the test on two drawn
rotor poles was readily available. Another component of the comprehensive testing offered by
SENSOPLAN was the modal analysis of the end windings. This analysis looks at the stability and
integrity of end windings, an area of the stator subject to extreme loads. On the basis of the results
obtained, SENSOPLAN drew up a condition analysis and risk evaluation and made recommendations
which would answer the operator’s questions concerning long-term availability. The operator now
knows: How long can the individual machines be run safely? Where is there a need for refurbishment?
When is the optimum time for a refurbishment? And what sourcing is required for replacements? A
reliable condition report requires specific expertise to know where and what to look for precisely during
analysis. SENSOPLAN can rely on extensive experience obtained from working on numerous large
machines. This is a prerequisite for recommending practicable measures.
SENSONEWS © SENSOPLAN 11/2006
SENSOPLAN AG Gewerbestr. 6
79801 Hohentengen a.H., Germany
Phone +49 7742 9298-0
Fax +49 7742 9298-10
SENSOPLAN Inc. 1020 e Wall Street
Mt. Pleasant, SC 29464, USA
Phone +1 843 329 0275
Fax +1 843 329 0956
Please visit us at www.sensoplan.com or send us a note to info@sensoplan.com (Europe) or sales@sensoplan.com (USA)
SENSO - News
SERVICE:
Resin Pressure Injection – Restoration of stator slot assembly
on generators subject to “vibration sparking”
The stator of a “global VPI” generator is immersed completely in an epoxy resin bath in the vacuum
pressure impregnation process (VPI) after the lamination stack and windings have been installed
during the production process. While this process has been used successfully on many generators in
the industry, there are certain models and production series of such “globally impregnated” generators
that exhibit a generic failure mechanism called “vibration sparking”:
Due to relative movements, settling effects and manufacturing tolerances of bars and lamination
stacks, and shrinkage of the impregnation resin, tiny gaps between the stator winding bar and the
lamination stack can occur in the slot. Once these tiny gaps have formed, further degradation occurs
as the low-energy slot discharge burns off more of the resin film until, finally, the bar is mechanically
loosened and begins to vibrate tangentially in the slot. This vibration causes an irregular break of the
electrical contact between the bar and the lamination (i.e. the slot assembly or coupling). The result is
that the vibrating bar acts like a “switch”, preventing the continuous electrical connection of the slot
corona protection to the lamination and the continuous flow of equalizing currents. When the “switch is
opened” and the current flow is severed, high energy sparks are created – hence the name “vibration
sparking”. Once this occurs, it does not take long for these sparks to destroy the corona protection
and, if countermeasures are not taken quickly, the main insulation will become damaged. In the worst
case, the winding needs to be replaced. Typically, such damage will occur after approx. 15,000 –
35,000 hours of operation at both ends of a generator and progress towards the center during further
operation.
By taking corrective action early on with the SENSOPLAN Resin
Pressure Injection (“RPI”) service, the damaging effects of vibration
sparking can be diagnosed and remedied prior to the onset of any
irreparable damage. The first step in the repair process is an
extensive and thorough inspection of the winding during a short
outage (cooler drawn for access, rotor stays in position). By using
endoscopy (boroscope) in the cooling air ducts, the SENOSPLAN
engineer is able to determine whether damage has taken place in the
slot assembly of the stator bars and assess the extent of this
damage. In a second step, the actual repair is conducted, preferably
during a routine major overhaul of the generator. Once the rotor has
Injection of resin
been removed, the stator winding is examined carefully and the
repair work adapted precisely to the specific condition. The SENSOPLAN laboratory manufactures a
semi-conductive resin compound designed specifically for this purpose. The appropriate viscosity and
electrical conductivity values, as well as the mechanical properties of the resin, are key to the success
of the repair. The resin is pressed into the cooling air slots to the right and the left between the bar and
lamination stack with a special injection tool, thus restoring the integrity of the stator slot assembly.
The SENSOPLAN engineer must apply precisely the exact amount of resin at each injection point.
Results: The assembly of the stator bars in the slot is restored to an acceptable, long-term operating
condition; the bar is once again securely fixed in place mechanically, and the requisite electrical
properties of the slot clearance are re-established.
To validate the success of the restoration, SENSOPLAN conducts
on-line partial discharge measurements on request, prior to and
following the repair work. The customer benefits: Lengthy
downtime is avoided; by correcting the slot assembly, the vibration
sparking effect is eliminated, thus significantly increasing the
service life of the stator winding. Professional and fast execution of
the work is guaranteed. In the framework of a unit outage with
SENSOPLAN as the general contractor, the RPI service represents
a cost-efficient method of eliminating this stator winding problem.
Cleaning of stator bore after repair
SENSONEWS © SENSOPLAN 11/2006
SENSOPLAN AG Gewerbestr. 6
79801 Hohentengen a.H., Germany
Phone +49 7742 9298-0
Fax +49 7742 9298-10
SENSOPLAN Inc. 1020 e Wall Street
Mt. Pleasant, SC 29464, USA
Phone +1 843 329 0275
Fax +1 843 329 0956
Please visit us at www.sensoplan.com or send us a note to info@sensoplan.com (Europe) or sales@sensoplan.com (USA)
Offering
SENSOPLAN – we have got the right solution:
Generator-Services:
Stator:
• Stator Vibration Services: Analysis and elimination of stator frame vibration problems
• SENSOCORE HI Checkup: Reliable on-site diagnosis of the generator stator lamination
stack using a high induction loop test under close-to-operating conditions
• SENSOCORE-Inject®: Patented repair of lamination stack hot spots in deep layers
• Stator End Winding Vibration Services: Stator end winding vibration problems solved
in a responsive and qualified manner
• Resin Pressure Injection RPI: Repair of the coupling between stator winding bar and
slot wall by injection of resin
• Stator Rewind Services: Dedicated improvement of the stator winding design
Rotor:
• Rotor Vibration Services: Rotor vibration problems solved in a responsive and qualified
manner
• Rotor Rewind Services: Dedicated improvement of the rotor winding design
• Top Tooth Cracking: Modification of the rotor shrink fit area
• Hydrogen Sealing Contour: Modification of the hydrogen sealing rings
Turbine-Services:
• Analysis and elimination of casing and shaft vibration problems
• Determination of residual life of steam turbines
• Steam Path Audit
Would you like more information?
Please contact:
Europe
SENSOPLAN AG
Thorsten Wasmuth, +49 7742 9298-16
thorsten.wasmuth@sensoplan.com
USA
SENSOPLAN Inc.
Brian Clark, +1 843 329 0275
brian.clark@sensoplan.com
SENSONEWS © SENSOPLAN 11/2006
SENSOPLAN AG Gewerbestr. 6
79801 Hohentengen a.H., Germany
Phone +49 7742 9298-0
Fax +49 7742 9298-10
SENSOPLAN Inc. 1020 e Wall Street
Mt. Pleasant, SC 29464, USA
Phone +1 843 329 0275
Fax +1 843 329 0956
Please visit us at www.sensoplan.com or send us a note to info@sensoplan.com (Europe) or sales@sensoplan.com (USA)